What is the difference between an asteroid and a planet?
Planets are big, round and have cleared their paths around the Sun. Asteroids are much smaller, usually lumpy, and share their paths with many others.
Planets and asteroids both travel around the Sun, but they are very different.
Planets are big. Even the smallest planet, Mercury, is about 4,880 kilometres across. Planets are so heavy that their gravity pulls them into round balls. They have also swept their paths around the Sun clear of most other objects. There are eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Asteroids are much smaller. Most are only a few kilometres across or less, and many are smaller than a football pitch. The biggest, Ceres, is about 940 kilometres across, and it is so big and round that it is called a dwarf planet. Most asteroids are lumpy, because their gravity is too weak to make them round. Millions of asteroids share the same region of space, in the asteroid belt between Mars and Jupiter.
Asteroids are leftover pieces from when the planets were forming. In the asteroid belt, Jupiter's strong gravity stirred them up, so they never joined together to make a planet. They crash into each other instead, breaking off pieces. Some of those pieces become meteorites.
So, a planet is like a finished cake, and asteroids are like the crumbs left on the plate.